The present invention relates to single-phase magnesium calcium thioaluminate phosphors, particularly thin film magnesium calcium thioaluminate phosphors for electroluminescent displays and more particularly for electroluminescent displays employing thick film dielectric layers. The phosphor emits green light. The invention also relates to an electroluminescent apparatus employing such phosphors. The phosphors may be deposited at low temperatures and subsequently annealed to develop optimum phosphor performance. The deposition temperature is sufficiently low to prevent damage to photoresist materials that are required to be in place during the phosphor deposition process. The annealing step may be done following lithography steps required to define the phosphor pattern for the sub-pixels.
In particular, the present invention relates to a high luminance green phosphor material for full colour electroluminescent displays that is compatible with photolithographic processes used to pattern thin phosphor films comprising distinct materials for red, green and blue sub-pixels on the display. Preferably, such electroluminescent displays employing thick film dielectric layers with a high dielectric constant.
Thick film dielectric structures provide for superior resistance to dielectric breakdown, a reduced operating voltage as well as the use of thicker phosphor films as compared to thin film electroluminescent (TFEL) displays, as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure when it is deposited on a ceramic or other refractory substrate will withstand somewhat higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. With these advantages and with recent advances in blue-emitting phosphor materials, these displays have approached the luminosity and colour coordinates required to achieve the technical performance of traditional cathode ray tube (CRT) displays. Nevertheless, achievement of optimum energy efficiency requires that different phosphor materials be used for red, green and blue sub-pixels for the displays. Thus, methods of patterning deposited phosphor films to form the respective sub-pixels are required. Such patterning is typically achieved using photolithographic processes, which require the use of polymeric photoresist materials that will only withstand relatively low temperatures during processing.
Traditionally, green sub-pixels for electroluminescent displays have used a manganese-activated zinc sulphide phosphor, and the use of a suitable optical filter to achieve acceptable CIE colour coordinates. However, because the spectral emission from this phosphor material is relatively wide, ranging from red to green, there is a substantial loss of luminance as the light passes through the filter, resulting in a relatively low energy efficiency and correspondingly high power consumption for the display. Alternate green phosphors with a narrower emission spectrum tuned to provide acceptable green colour coordinates have been evaluated. One such phosphor is terbium-activated zinc sulphide that has a spectral emission centered on green, but this phosphor still requires use of a filter to provide acceptable colour coordinates. Another phosphor is europium-activated calcium thioaluminate, which has a green emission that can provide acceptable colour coordinates without the use of an optical filter. However, this phosphor is more difficult to deposit as a thin film for electroluminescent displays. A process for depositing such a phosphor film is described in U.S. patent application Ser. No. 09/747,315 but the process requires that deposition be carried out using a substrate temperature of at least about 200xc2x0 C. followed by an annealing process at a temperature of 650xc2x0 C. in order to achieve adequate luminosity. The required thioaluminate compound is not formed from the deposited materials during the deposition step, and the annealing step is required to form the thioaluminate. The maximum annealing temperature is determined by the thermal stability of the substrate upon which the phosphor has been deposited.
In general, both the deposition temperature and the annealing temperature are important variables in determining the process conditions for a phosphor film with optimum performance. If the deposition temperature must be lowered due to problems of stability of the deposition materials at higher temperatures, it may not be possible to compensate for the lower deposition temperature by use of a higher annealing temperature. The reasons for this are complex, but generally relate to the formation of intermediate compounds following deposition but prior to annealing. The substrate temperature during deposition may affect the nature and concentration of these intermediate compounds, which may in turn affect the quality of the final phosphor film following annealing.
In full colour electroluminescent displays with a thick film dielectric employing a patterned phosphor structure, the maximum temperature tolerated by photoresist materials used in the patterning process may limit the deposition temperature. The annealing step is typically done after completion of the patterning process when photoresist materials have been removed. The maximum temperature that can be tolerated by the thick film dielectric structure determines the maximum temperature acceptable in the annealing step.
The use of calcium thioaluminate phosphors is taught in U.S. patent application Ser. No. 09/747,315, with the phosphor being deposited using electron beam evaporation from two sources. A U.S. patent application entitled xe2x80x9cThioaluminate Phosphor Material with a Gadolinium Co-activatorxe2x80x9d and filed May 30, 2001 teaches the use of gadolinium as a co-activator to improve the luminosity and electro-optical properties of electroluminescent devices of phosphors that include calcium thioaluminate.
A green emitting phosphor that may be deposited at relatively low temperatures, especially at temperatures that are sufficiently low to reduce or prevent damage to photoresist materials used in preparation of electroluminescent devices, would be useful.
An aspect of the present invention provides a thin film phosphor for an electroluminescent device, said phosphor comprising a compound of the formula MgxCa1-xAl2S4:M, where the value of x is in the range 0 less than x less than 0.3 and M is a rare earth activator.
In a preferred embodiment of the thin film phosphor, the value of x is in the range 0.05 less than x less than 0.20.
In another embodiment, said rare earth activator is europium or cerium, especially europium.
In a further embodiment, the atomic ratio of europium to aluminum is in the range of 0.005 to 0.05.
Another aspect of the present invention provides an electroluminescent device comprising the thin film phosphor on a substrate.
In a preferred embodiment, the phosphor has a thickness in the range of 0.2 to 1.0 micrometers, especially a thickness in the range of 0.3 to 0.6 micrometers.
In further embodiments, the thin film phosphor is adjacent to a thin film of zinc sulphide, especially sandwiched between thin films of zinc sulphide.
A further aspect of the present invention provides a thin film phosphor for an electroluminescent device, said phosphor comprising magnesium calcium thioaluminate activated with a rare earth metal, said magnesium calcium thioaluminate containing an amount of magnesium to effect a lowering of the temperature of deposition of the phosphor on a substrate.
In preferred embodiments of the method, the amount of magnesium is sufficient to lower the temperature of deposition to not more than 200xc2x0 C., especially sufficient to lower the temperature of deposition to not more than 175xc2x0 C.
Another aspect of the present invention provides a method for the preparation of a phosphor on a substrate, said phosphor comprising a compound of the formula MgxCa1-xAl2S4:M, where the value of x is in the range 0 less than x less than 0.3 and M is a rare earth activator, said method comprising the steps of:
(i) depositing a mixtures of sulphides of magnesium, calcium, aluminum and said rare earth metal on a substrate, the ratios of said sulphides being selected to provide a mixture of sulphides of said formula on the substrate; and
(ii) annealing the mixture of sulphides on the substrate so as to form said phosphor.
In a preferred embodiment of the method, the mixture of sulphides is deposited on the substrate at a temperature of not greater than 200xc2x0 C., especially not greater than 175xc2x0 C., and preferably not greater than 150xc2x0 C.
In another embodiment, prior to step (i), a photoresist pattern is deposited on said substrate, especially using photolithography.
Still another aspect of the invention provides a method for the preparation of a phosphor on a substrate, said method comprising the steps of:
(i) depositing a photoresist pattern on said substrate;
(ii) depositing a mixture of sulphides on said photoresist pattern at a temperature of not greater than 200xc2x0 C., said mixture of sulphides forming magnesium calcium thioaluminate;
(iii) removing said photoresist; and
(iv) annealing said mixture of sulphides to form said phosphor.
In a preferred embodiment of the method, the phosphor comprises a compound of the formula MgxCa1-xAl2S4:M, where the value of x is in the range 0 less than x less than 0.3 and M is a rare earth activator, said mixture of sulphides being selected to form said phosphor.
In further embodiments, the temperature in step (ii) is not greater than 175xc2x0 C., especially not greater than 150xc2x0 C.